Stray light in an optical imaging system, be it a rifle scope, spotting scope, or CBO system, can be a result of internal reflections off surfaces within that imaging system from sources outside of the designed-for-field-of-view. This is illustrated by the light ray-trace diagram for a typical riflescope shown in FIG. 1. In that diagram, the rifle scope optics 10 comprise an objective lens system 12, a relay lens system 14, and an ocular lens system 16, which present the transformed incoming bundle of rays 18 from a distant object along the optical axis 20 of the imaging system to the user's eye 22. (The term “lens” is used herein to refer to one or a combination of two or more lens elements, while the term “lens system” is used only to refer to a combination of two or more lens elements.) However, off-axis stray light rays 24 that enter the system 10 through the objective lens system 12 can cause spurious reflections within the imaging system that also reach the user's eye through the ocular lens system 16. When such off-axis sources are particularly bright, the glare produced can compete with the light of the on-axis image, resulting in degradation of, or an inability to see, images of objects of interest presented to the eye. It would be desirable to be able to block this off-axis light and thereby preclude it from reaching the eye or, in the case of a CBO imaging system, the camera detector.
Many DVO and CBO systems are typically used outdoors. In such use it is particularly desirable to protect the exposed objective and ocular lenses from environmental degradation. This is because the objective lens is typically the largest, most expensive and, usually, most vulnerable lens in an imaging system. But, in addition, localized imperfections in the ocular lens system, e.g., chips in a lens, mud on a lens surface, and other physical variations from the imaging system design can dramatically impact the ability of DVO system to image the field angle to which the imperfection corresponds. Consequently, it is desirable to be able to protect entirely these optical surfaces from hazards in the environment.
When an imaging system is used in a military environment, a law enforcement environment or the like, the optical “signature” that such a system presents to persons other than the user, that is, light reflected from the system, is especially important. Reflections, e.g., sun “glint,” from the objective lens or interior surfaces of the imaging system, represent a signature that can be seen or detected by an adversary. The magnitude of the optical return signature is proportional to the cross sectional area of the optical component. This means, for example, that by reducing the optical clear aperture diameter of the imaging system by a factor of 1/X, the optical signature power is reduced by a factor of 1/X2 (where X>1). Because of this characteristic, it would be desirable to have a variable diameter aperture disposed in front of the objective lens that enables an operator to minimize this optical signature power, while simultaneously balancing that desire against the quality of the image and exit pupil diameter produced to the user.
At night, military and law enforcement situations often require the use of an “in-line” night vision device with an optical imaging system adapted to image relatively long wavelength light that is not significantly visible to the human eye. For example, as shown in FIG. 2, a night vision device 26 may be disposed in front of the objective lens 28 of a DVO system, specifically a rifle scope 30, to present to the DVO a visible-light representation of an infra-red image. Such devices typically are afocal, meaning they optimally present an image of nominally collimated light to a wavelength-shifting display and optical amplifier 32 to produce a visible collimated light to the rifle scope 30. A hazard of using such an imaging system, is that the relatively bright display represents a source of visible light 34 which can be reflected off of the convex objective lens 28 of the rifle scope 30, or exterior components of the system, and re-directed toward an adversary as shown by ray 36.
Further, it is not uncommon to use thermal in-line night vision devices with telescopic optical imaging devices during the day for target detection. Such devices image mid-to-long wave infrared radiation that is a function of temperature only, not ambient visible illumination. But the magnifying lenses that collimate the visible display, for presentation to the DVO, are typically smaller and have a narrower field angle than the DVO to which they are coupled. This presents a problem, because the DVO objective lens can collect visible light within its wider field of view, in addition to the magnified display designed to be seen, as illustrated by light ray 38 in FIG. 2. This competing information, that is, thermal light converted to visible light and direct visible light from around the edges of the thermal imaging system, represents a degradation in imaging performance.
For such night and day applications of an in-line night-vision device, in conjunction with a DVO system, it would be desirable to have a means of preventing visible light from exiting (night) and entering (day) the optical train of the entire imaging system.